The present invention relates to polymeric microparticles capable of promoting the crosslinking reaction of a crosslinkable polymer used in, for example, coating compositions, printing inks, sealants, adhesives and the like.
As is well-known, melamine-acrylic coating compositions are cured into a rigid coating film through the crosslinking reaction of hydroxyl group-containing acrylic polymer with melamine resin at an elevated temperature. Usually a catalytic amount of proton donors is added to the reaction system to promote the crosslinking reaction. Otherwise the reaction will not be completed within a reasonable length of time.
Other examples of such agents which are capable of promoting the crosslinking reaction of crosslinkable polymers include acid catalysts and tertiary amines to be incorporated to epoxy resins, organic tin compounds and tertiary amines used in polyisocyanate-containing systems, organic cobalt salts used in unsaturated polyesters, dryers used in drying oils and other oxidation-polymerizable resins, and photosensitizers used in various photocurable compositions.
The term "crosslinking reaction" as used herein means the reaction in which starting monomers, oligomers or polymers are reacted with each other or with a crosslinking agent to increase their molecular weights by setting up chemical links between the molecular chains of the resulting polymers.
Crosslinking reaction promoters are used not only in coating compositions but also in printing inks, adhesives, sealants and other polymeric products. Although its presence is practically imperative, the crosslinking reaction promoter is a separate component for the polymer to be crosslinked and is often incompatible with the polymer in a system containing the same. Thus, it is often difficult to disperse the crosslinking reaction promoter uniformly in the polymer system for ensuring uniform crosslinking. Also, the polymer system containing the crosslinking reaction promoter generally tends to be unstable upon storage.
The crosslinking reaction promoter usually remains as such in the cured polymer after having performed its function and is susceptible to migration and leaching outside the cured polymer, thereby causing various disadvantages.
It is, therefore, a principal object of the present invention to provide a new form of crosslinking reaction promoters which may avoid or minimize various disadvantages associated with conventional crosslinking reaction promoters.
It is another object of the present invention to provide a method for curing a crosslinkable polymer through a crosslinking reaction into a rigid state in which the crosslinking reaction promoter is immobilized.
Other objects and advantages of the present invention will become apparent as the description proceeds.